[UK-CONTEST] Current transformers on vertical antennas

[G3SJJ]

I am just about to start a rebuild of my phased pair of verticals for 
40m plus also add a 5/8w for 30m with an increased emphasis on 
efficiency. In the past I have experienced RF feedback on 40m SSB and 
wondered if part or all of the problem is lack of current chokes where 
the feedlines connect to each element. To this end I have decided to 
anyway add chokes.

ON4UN's Low Band Dxing suggests using between ten and thirty 43-1024 
beads which will slip over RG213. I guess I could look at the data sheet 
but wondered if anyone had an instant answer to the optimum number for 
7MHz and also 10.1MHz?

Chris G3SJJ